DWI shows high contrast between tumor and surrounding tissue, which shows potential for target volume delineation in head-and-neck radiotherapy treatment planning. In this study we assess the performance of a diffusion weighted fast spin echo sequence (DW-SPLICE) for target delineation in terms of interobserver agreement and spatial concordance with PET. Fifteen patients underwent both PET and DW-SPLICE. PET was segmented using a Gaussian mixture model, DW-SPLICE was delineated by 3 observers. Target volume delineation using DWI is feasible in head-and-neck radiotherapy. Using an optimized DWI sequence, target volumes could be defined with good interobserver agreement and large similarity with PET.